The ever increasing human life span poses a continuous demand to the pharmaceutical and biotechnology market for improving the therapeutic efficacy of drugs. However, there are major drawbacks associated with conventional dosage forms, such as fast renal clearance of small drugs and enzymatic degradation of biopharmaceuticals.
Common problems encountered in the use of small molecule drugs, especially for anti-tumor therapeutics, are their low solubility, rapid excretion and untargeted systemic biodistribution leading to severe toxic side effects.
Although more and more biopharmaceuticals are in development, many of these pharmaceuticals have problems that are typical of polypeptide therapeutics, such as short circulating half-life, immungenicicty, proteolytic degradation, and low solubility.
Several strategies have emerged as ways to improve the pharmacokinetic and pharmacodynamic properties of pharmaceuticals. Examples of such strategies are; manipulation of the amino-acid sequence of biopharmaceuticals to decrease immunogenicity and proteolytic cleavage; fusion or conjugation of small molecules or biopharmaceuticals to immunoglobulines and serum proteins, such as for example albumin; incorporation of pharmaceuticals into drug delivery verhicles for protection and slow release; and conjugating to natural or synthetic polymers.
The most commonly used polymer conjugation strategy is the coupling of poly(ethylene glycol) (PEG) to the small molecule pharmaceutical or bio-pharmaceutical. Important precedents of PEGylation already demonstrated improved therapeutic efficacies leading to commercial applications of PEGylated bio-pharmaceuticals, such as enzymes, cytokines and antibodies, as well as a PEGylated liposome carriers for the anticancer drug Doxorubicin.
PEG is an attractive polymer for drug conjugation since it increases the hydrodynamic radius of a drug and shields it, at least partially, from interactions with the body, including the immune system and proteolytic enzymes. Especially this latter shielding property is the main driving force for the success of PEG, compared to many other water soluble polymers, which is believed to result form the good hydration of PEG. By increasing the molecular weight of a molecule through PEGylation, several significant pharmacological advantages over the unmodified form arise, such as an improved drug solubility, extended circulating half-life, reduced immunogenicity, increased drug stability and an improved protection against proteolytic degradation.
Despite the common use of PEGylation there are several disadvantages associated with its use. Sometimes, hypersensitivity and the formation of PEG antibodies is observed. It is also observed that when PEG with high molecular weights is used, it accumulates in the liver, leading to the so called macromolecular syndrome. The chain length of the PEG molecules may be reduced under the influence of enzymes, such as P450 or alcoholdehydrogenase, giving rise to toxic side products.
With respect to small therapeutic molecules it is often observed that with PEG only a relatively low drug loading can be achieved due to the presence of merely one or two hydroxyl terminal groups that can be activated. Furthermore, orthogonal functionalization of PEG or PEG dendrons with the therapeutic moiety, detection moiety or targeting moiety is not readily possible.
Furthermore, it is relatively difficult and hazardous to prepare PEG as explosive and highly toxic condensed ethylene oxide monomers are required. In addition, PEG has a limited storeability, i.e. an antioxidant is required for storage in order to avoid peroxide formation.
GB 1 164 582 describes a polyalkylenimine/polycarbonamide graft copolymer having as the backbone a linear polyimine wherein the imino nitrogen atoms are separated on the average by no more than 5 carbon chain atoms, having pendant from some or all of such nitrogen atoms polycarbonamide side chains having as repeating unit a carbonamido divalent radical with a chain of 3 to 18 carbon atoms between the amido nitrogen atom and the carbonyl group, said side chains having a number average molecular weight of at least 500.
WO 2009/112402 describes a heterofunctional polyoxazoline derivative of the general structure: R1— {[N(COX)CH2CH2]o-[N(COR2)CH2—CH2)]n-[N(COY)CH2—CH2)]m}a—Z wherein:                R2 is independently selected for each repeating unit from an unsubstituted or substituted alkyl, an unsubstituted or substituted alkenyl, an unsubstituted or substituted aralkyl or an unsubstituted or substituted heterocyclylalkyl group;        X is a pendent moiety containing a first functional group;        Y is a pendent moiety containing a second functional group;        and m are each an integer independently selected from 1-50;        n is an integer selected from 0-1000.        
WO 2009/112402 describes a random copolymer of linear polyethylenimine comprising two monomeric units, including a monomeric unit comprising a pendant moiety having the following formula: —CH2CHR1CONHCHRaRb;
wherein:
                R1 represents a hydrogen atom or a C1-C6 alkyl group,        Ra represents CH2-imidazole;        Rb represents COOR2;        R2 represents a hydrogen atom, a C1-C6 alkyl, aryl or aralkyl group, in which the alkyl group is a C1-C6 alkyl,        
n is a number comprised between 1 and 99% of the total monomers and
m is a number comprised between 1 and 99% of the total monomers.
The international patent application also describes target molecule-polyoxazoline conjugates comprising a target molecule wherein the target molecule is a therapeutic moiety, a diagnostic moiety or a targeting moiety.
JP 2005161698 describes a recording material that is formed by providing a recording layer including an acylhydrazide compound and a diazo compound. The Japanese application contains a formula A40 that represents a 2-substituted oxazoline, wherein the 2-substituent is represented by —CH2—CONH—NHX, wherein X is a p-substituted benzyl group.
Zarka et al. (Amphiphilic Polymer Supports for the Asymmetric Hydrogenation of Amino Acid Precursors in Water, Chem. Eur. J. 2003, 9, 3228-3234) describe the synthesis of amphiphilic, water-soluble diblock copolymers based on 2-oxazoline derivatives with pendent (2S,4S)-4-diphenylphosphino-2-(diphenylphosphinomethyl) pyrrolidine units in the hydrophobic block. The synthetic strategy involves the preparation of a diblock copolymer precursor with ester functionalities in the side chain; which were converted into carboxylic acids in a polymer-analogous step and finally reacted with the PPMligand in the presence of dicyclohexylcabordiimide yielding an tertiary amide linkage in between the polymer and the ligand.
Cesana et al. (First Poly(2-oxazoline)s with Pendant Amino Groups, Macromol. Chem. Phys. 2006, 207, 183-192) describe the preparation of a poly(2-oxazoline) with pendant amino groups starting from 2-oxazoline monomer with a Boc protected amino function, 2-[N-Boc-5-aminopentyl]-2-oxazoline (Boc-AmOx). This monomer could be converted via living cationic ring-opening polymerization to homopolymer. After quantitative deprotection, poly(2-oxazoline)s with pendant amino functions were obtained. Copolymerization with different monomer ratios of Boc-AmOx and 2-ethyl-2-oxazoline (EtOx) was performed. A cross-linking reaction with a bifunctional isothiocyanate (Ph(NCS)2) resulted in poly(2-oxazoline) hydrogels.
Luxenhofer (Thesis: Novel Functional Poly(2-oxazoline)s as Potential Carriers for Biomedical Applications, Technische Universität München (2007)) describes a poly(2-oxazoline) comprising 20 units of 2-methyl-2-oxazoline and 5 units of 2-aminoethyl-2-oxazoline. This polymer was cross-linked with hexamethylene diisocyanate to produce an amine hydrogel.
Liu et al. (Shell Cross-Linked Micelle-Based Nanoreactors for the Substrate-Selective Hydrolytic Kinetic Resolution of Epoxides, J. Am. Chem. Soc. 2011, 133, 14260-14263) describe amphiphilic poly(2-oxazoline) triblock copolymers. These copolymers are prepared from a monomer mixture that includes a cinnamate-functionalized oxazoline monomer that was prepared by a five step synthetic procedure yielding a 2-oxazoline monomer with a C11 spacer in between the monomer ring and the cinnamate.
The polyoxazoline polymers with pendant amine containing units reported by Cesana et al. and Luxenhofer suffer from the disadvantage that the amine groups need to be protected. Furthermore, these protected amine-groups tend to interfere with the polymerization process, yielding rather poorly defined polymers. This latter disadvantage can be overcome by replacing the common initiators with a preformed oxazolinium triflate initiating moiety further complicating the polymerization process.
Conversion of the pendant amines in the aforementioned polyoxazoline polymers into an amide moiety yields functionalized polymers similar to those described by Liu et al. A disadvantage of these amide-containing polymers is that a primary amine is retained after hydrolysis, i.e. biodegradation, giving rise to toxic polymers.
In view of the problems mentioned above a need exists for polymers with excellent conjugating properties, causing less side effects and providing excellent delivery of small therapeutic molecules and biopharmaceuticals to the tissue involved. In addition, there is a need for a simple process for synthesizing these polymers in good yield and in a well-defined form.